1. The Field of the Invention
This invention relates to electronic filter circuits and more particularly in electronic circuits for filtering electrical wave forms where information is contained in patterns and time spacing of zero-voltage crossings.
2. Description of the Prior Art
Many present day data processing and computer systems utilize a magnetic memory in which digital information is written onto a magnetic disk that is rotated at high speeds. In such magnetic memory systems, the rotating disk is coated with a magnetic material and digital information is encoded on the disk surface by selectively magnetizing the portions of the surface by means of an electromagnetic transducer arrangement. Digital information is normally recorded in a serial track on the rotating disk surface as it passes beneath the recording transducer. Typically, a magnetic encoding pattern is used in which the direction of the magnetic field changes rapidly along the length of the track. This rapidly changing magnetic pattern is called a "flux reversal," and the digital information is normally encoded in the spacing and patterns of the flux reversals.
Digital information is normally read out of a magnetic disk memory by placing a second magnetic transducer close to the magnetic disk surface. As the flux reversals recorded on the disk surface move past the transducer a voltage waveform is induced in the transducer which waveform has a series of positive and negative peaks.
The digital information is then recovered from the read-voltage waveform by differentiation. Voltage peaks in the original waveform are converted by differentiation into zero-voltage crossings since the derivative of the read-voltage waveform never goes to zero except at the peaks. The zero-voltage crossings may be easily detected by well-known means and used to develop a waveform representative of the stored digital information.
Such a magnetic recording system provides for substantial noise immunity, because noise which does not produce a false zero crossing of the read-signal derivative has almost no effect. However, with some data patterns and particularly with high resolution read transducers, the slope of the read-signal voltage between peaks is flat enough so that its derivative has a very low amplitude. At this low point of the derivative the signal is particularly susceptible to noise which causes false zero crossings.
Prior art magnetic disk systems often identified valid and invalid zero crossings by means of amplitude discriminating circuits. However, a problem in the prior art arose because amplitude discrimination circuits had to have high resolution in order to properly discriminate between invalid zero crossings and valid pulses with low amplitude. Such circuits were, therefore, complicated and expensive.